title: Field camera measurements of gradient and shim impulse responses using frequency sweeps.
abstract: OBJECTIVE
  Applications of dynamic shimming require high field fidelity, and characterizing the shim field dynamics is therefore necessary. Modeling the system as linear and time-invariant, the purpose of this work was to measure the impulse response function with optimal sensitivity.
  
  
  METHODS
  Frequency-swept pulses as inputs are analyzed theoretically, showing that the sweep speed is a key factor for the measurement sensitivity. By adjusting the sweep speed it is possible to achieve any prescribed noise profile in the measured system response. Impulse response functions were obtained for the third-order shim system of a 7 Tesla whole-body MR scanner. Measurements of the shim fields were done with a dynamic field camera, yielding also cross-term responses.
  
  
  RESULTS
  The measured shim impulse response functions revealed system characteristics such as response bandwidth, eddy currents and specific resonances, possibly of mechanical origin. Field predictions based on the shim characterization were shown to agree well with directly measured fields, also in the cross-terms.
  
  
  CONCLUSIONS
  Frequency sweeps provide a flexible tool for shim or gradient system characterization. This may prove useful for applications involving dynamic shimming by yielding accurate estimates of the shim fields and a basis for setting shim pre-emphasis.
authors.affiliation: Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland.
authors.forename: S Johanna
authors.initials: SJ
authors.lastname: Vannesjo
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Dietrich
authors.forename: Matteo
authors.initials: M
authors.lastname: Pavan
authors.forename: David O
authors.initials: DO
authors.lastname: Brunner
authors.forename: Bertram J
authors.initials: BJ
authors.lastname: Wilm
authors.forename: Christoph
authors.initials: C
authors.lastname: Barmet
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
mesh_terms: D000465:Algorithms; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007090:Image Interpretation, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: dynamic field camera; dynamic shimming; eddy currents; gradient characterization; impulse response function; magnetic field monitoring; shim characterization